The use of polypropylene (PP) in pure form and also as a rubber modified, impact resistance adjusted version (PP-EPDM blends) for workpieces has been steadily increasing in a number of areas of application, in particular due to its excellent properties as compared with other plastics. In the automobile sector especially, PP has become the choice for internal and external automobile parts, it being used for instance for bumpers, wing-mirror holders, door handles, etc.
When using PP add-on pieces in external fields of use, it is often desirable, and it is happening more and more frequently, that the workpieces are lacquered for decorative and protective reasons. Unfortunately, due to the only very low surface tension and low polarity of PP, it is difficult to produce both adequate coating film formation and also sufficiently good adhesion to the substrate. This problem can be solved on an industrial scale by, for instance, physical-chemical pretreatment processes on PP, such as, for instance, flame treatment or low pressure plasma activation. In both methods, a polar substrate surface is produced which can then be coated without any problem. The surface activation produced in this way, however, tends to reverse and in addition these activation processes require expensive equipment. Some processes can only be operated batchwise (plasma) while some result in defective spots when the workpieces have a complex shape (flame treatment).
Another solution to the problem is the use of coating systems, such as, for instance, the use of chlorinated polyolefins (CPO) dissolved in organic solvents, wherein the CPO is used either on its own (e.g. U.S. Pat. No. 4,303,697, U.S. Pat. No. 4,070,421 and U.S. Pat. No. 3,579,485) or combined with a film-forming polymer such as e.g. acrylate or urethane polymers. The CPO produces outstanding adhesion to PP substrates. Chlorination itself is required in order to ensure adequate solubility in the solvents used such as, for instance, toluene or xylene. The relatively poor compatibility of CPO with conventional binder resins, however, is a disadvantage.
All these coating systems, however, have the serious disadvantage of the presence of very large amounts of volatile, mostly also aromatic, solvents which ought to be drastically reduced for reasons which relate to the environment and occupational safety.
Furthermore, WO 93/01 244 describes aqueous compositions which consist of chlorinated polyolefins (CPO) and a high proportion of a non-ionic emulsifier as well as amines and water for neutralisation purposes. These compositions may be used as primers or optionally stirred into the paint.
WO 90/12 056 also describes aqueous, generally solvent-free, compositions based on CPO and anionic emulsifiers and water, which are produced by a special process. Coating agents may optionally be produced from these compositions by the addition of polyurethane resins. These polyurethane resins are described as anionically stabilised resins.
WO 90/12 656 describes aqueous coating agents based on low molecular weight alcohols, chlorinated resins and emulsifiers and amines acting as emulsifiers. These coating agents are intended to have a solids content of less than 5 wt. %. They are applied directly to the substrate being coated.
The aqueous coating agents mentioned above are those which are applied to a plastics substrate, physically dried and then over-coated with further coating agents. They have the disadvantage that, after conventional drying of these systems, coatings are obtained which exhibit obvious weaknesses with regard to water resistance and adhesion to the substrate when they are subject to condensation or storage under wet conditions. This leads to defects in adhesion of the upper layers of lacquer which produces optical damage and also has an adverse effect on the mechanical stability of the workpiece.